Methylation of nicotinamide in rat liver cytosol and its correlation with hepatocellular proliferation

The changes in the activity of nicotinamide: S-adenosylmethionine methyltransferase (nicotinamide methylase) were studied in rat liver which was subjected to different rates of cellular proliferation. The cytosolic enzyme activity increased 3-4-fold in the first 24-48 h after partial hepatectomy and decreased again to the basal levels until 4 days post-operatively, whereas it remained unchanged in the livers of sham-operated animals. A single administration of thioacetamide at a dose of 50-250 mg/kg body weight, a treatment which induces hepatocellular proliferation as well, also enhanced the enzyme activity 2-3-fold 24 h after drug administration. This activity increase was associated with a marked lowering of intracellular NAD content of as much as 50% of the control levels. D-Galactosamine, a known hepatotoxic agent causing acute hepatitis in experimental animals and preventing DNA synthesis in regenerating liver, blocked the activity increase in regenerating rat liver. The rate of 1-methylnicotinamide synthesis, as measured by incubating liver slices in the culture medium supplemented with [14C]nicotinamide as a precursor, was found to be 2-4 times higher in the slices from regenerating liver and thioacetamide-treated rat liver than those from non-proliferating control liver. These results, together with our previous finding on the enhancement by 1-methylnicotinamide of the growth of cultured rat liver cells (Hoshino, J., Kühne, U. and Kröger, H. (1982) Biochem. Biophys. Res. Commun. 105, 1446-1452), support the view that nicotinamide methylase and its product, 1-methyl-nicotinamide, are involved in the control of hepatocellular DNA synthesis and proliferation.